Python module vatnumber doesn't seem maintained anymore. Therefore, we should:
    - call directly stdnum (which is maintained and mostly used everywhere in vatnumber)

Also improve stdnum import, vat fix method and vat expected formats

task-1915371

closes odoo/odoo#36978

Signed-off-by: Quentin De Paoli (qdp) <qdp@openerp.com>